In current medical practice, at least for highly invasive surgery, a patient is placed under general anesthesia. Anesthesiology is a medical art practiced in the United States by and large by board certified physicians (anesthesiologists) and nurses (nurse anesthetists) specifically trained to administer anesthetic drugs and monitor patients under anesthesia. The state of patient anesthesia is attained by the controlled administration of various drugs with known anesthetic properties. These drugs cause the patient to lose consciousness, sensation, and motor control. The physician monitors the patient's state of awareness by means of a number of disparate clinical signs known empirically to provide useful and reliable information about the patient's state of unconsciousness.
Generally, the patient is anesthetized prior to surgery by the specialized medical practitioner (anesthesiologist or nurse anesthetist), who administers one or more vapors or gases by inhalation or introduces anesthetic drugs intravenously. Volatile substances include nitrous oxide, sevoflurane, desflurane, isoflurane, and halothane. Intravenous anesthetics include pentothal, propofol, methohexital, and etomidate.
A correctly administered general anesthetic should remove any sensation of pain and any awareness of the operation itself. (Patients insufficiently deeply anesthetized have reported terror at becoming aware of the surgical procedure while paralyzed.)
The anesthetic should further disable the patient's motor control so that the patient cannot move. Otherwise, the patient may exhibit involuntary (reflex) muscle movements, which can disturb the area being surgically manipulated. Prevention of movement can be accomplished by anesthetic agents acting on the central nervous system or with a blockade of the neuromuscular junction with muscle relaxants.
Finally, the anesthesia must avoid depressing the patient's blood pressure so much as to reduce blood flow to the brain to a dangerous extent. Generally 50 mm Hg for mean arterial pressure is a lower limit.
A trained anesthesiologist or nurse anesthetist will monitor the patient's vital signs such as respiration and pulse rates, check the patient's pupil dilation, and check certain reflexes, such as the lash reflex, and other physiological signs to estimate the depth of anesthesia. In some instances, however, either the practitioner does not have access to all of the required clinical information or other circumstances intervene. For example, in some procedures the patient is draped in such a way as to make observation of some clinical indicators difficult or impossible. In addition, in very lengthy procedures the attention of even the best practitioner can flag.
In such circumstances it would frequently be useful to have an electronic monitor to track the patient's level of consciousness. In particular, it sometimes would be useful to have an instrument, which, once the plane of anesthesia is established qualitatively by the anesthesiologist using traditional clinical indicators, would indicate significant changes in the patient's state of anesthesia or patient responses to stimuli, which would indicate insufficient anesthesia.
A number of inventors have developed systems for using EEG signals, generally in combination with other signals, to monitor anesthesia, sleep, or other states on the consciousness-unconsciousness continuum. Kaplan et al., U.S. Pat. No. 5,813,993, issued Sep. 29, 1998, disclosed a drowsiness detection system based on EEG signals. This invention relies heavily on frequencies in EEG signals above 30 Hz. It does not use any form of norming and in addition applies an ad hoc weighted sum of inverted spectral power coefficients. Maynard, U.S. Pat. No. 5,816,247, issued Oct. 6, 1998, uses a combination of time domain amplitude envelope analysis and frequency analysis in conjunction with a trainable neural network to classify awareness and sleep states. Kangas et al., U.S. Pat. No. 5,775,330, issued Jul. 7, 1998, uses transform processing and neural net analysis to classify states of anesthesia. The output of the neural net could be used to produce a single index of awareness. However, all of these prior art systems either represent an unnecessary level of complexity or an absence of empirical basis or both.
A prior patent to John, U.S. Pat. No. 5,699,808, issued Dec. 23, 1997, discloses a system to monitor multiple patients simultaneously in the surgical recovery room or in intensive care. This system, however, combines certain features of EEG signals and other features including those of evoked potentials to arrive at an estimate of the patient's state of consciousness. It specifically incorporates the use of electrocardiograph (EKG) and electromyograph (EMG) electrodes and also input from a blood pressure detector and from a respiration monitor. This prior art system also requires evoked potentials, specifically Brainstem Auditory Evoked Response (BAER) and Brainstem Somatosensory Evoked Response (BSER). Use of evoked potentials, however, involves the use of additional disposables and a longer set-up time. Further, this system relies very heavily on self-norming and in particular on updating self-norming depending on the state of the patient.
An earlier patent to the same inventor, John, U.S. Pat. No. 4,557,270, issued Dec. 10, 1985, suffered from additional and more severe limitations since it required measurement of blood temperatures and volumes. Finally, John, U.S. Pat. No. 4,545,388, issued Oct. 8, 1985, disclosed the basic process of self-norming of processed EEG data.
Another inventor, Prichep, U.S. Pat. No. 5,083,571, issued Jan. 28, 1992, disclosed a significant advance in the utilization of EEG signals for diagnostic purposes. Prichep disclosed the use of discriminant analysis to sharpen the diagnostic capability of quantities derived from EEG signals with respect to certain well-known diagnostic categories of psychiatric illness. This work compared quantities derived from a patient with parameters derived from populations of persons thought to suffer from specific identified illnesses.
Finally, another patent issued to John, U.S. Pat. No. 6,067,467, issued May 23, 2000, applied discriminant analysis to the statistical differentiation of unconscious from conscious states from EEG signals. However, this invention relied heavily on BAER and BSER signals and self-norming. In addition, this invention stated, with respect to Chamoun, U.S. Pat. No. 5,010,891, issued Apr. 30, 1991, that "the comparison of patients with a normal group, in itself, is not believed to provide reliable information in the surgical context of determining if a patient will be sufficiently anesthetized." App. at p. 4. Since that time, however, the current inventors have learned from further investigation and experimentation that population-norming is sufficiently reliable and self-norming adds unnecessarily to the complexity of the system without adding to performance. What is therefore most lacking in all of these prior art inventions is simplicity and cost effectiveness.
It is therefore an object of the current invention to provide an EEG based anesthesia monitoring system that completely avoids use of transducers for and inputs from other than EEG signals, that is, avoids the use of pulse, blood pressure, and respiration rate sensors and leads. It is a further object of this invention to provide an anesthesia monitoring system based on EEG signals, which completely avoids the need for BAER and BSER stimulation and response monitoring. It is a further object of this invention to provide an EEG based anesthesia monitoring system which dispenses with the cumbersome and error prone process of self-norming. It is another object of this invention to provide an EEG based anesthesia monitoring system which utilizes sample population-norming.